two mcu's reading off single hall sensor

hall sensor is for detecting RPM, has Vcc, gnd, and return. Not sure about the specifics, but it goes to an OEM data logger.

I'd like to tap the output into an arduino for alternative purposes. I have reason to believe that the OEM DL runs at 5 volts already, but I don't know that for a fact.

My main question, will adding another detection circuit to the hall's output effect anything?

Should be OK. Arduino input takes in very little current. I suggest you get an oscilloscope to read the sensor first, just to make sure your 5v assumption is correct.